1. Technical Field
The present invention relates to a device for controlling the volumetric capacity of a screw compressor of the kind having a housing enclosing a compression chamber having an axis in which there are arranged two rotors which are in meshing engagement with each other, one end of the compression chamber being provided with a low-pressure end wall and the other end thereof being provided with a high-pressure end wall, a low-pressure port being arranged at the low-pressure end wall and a high-pressure port being arranged at the high-pressure end wall, a cylindrical, preferably rotary-symmetrical, bore being arranged with its longitudinal axis substantially parallel to the axis of the compression chamber and communicating with the compression chamber through a plurality of channels, disposed one after the other in the axial direction of the bore, a first end of the bore being connected to the low-pressure port, whereby a plunger is slidably arranged in the bore, the plunger closing a varying number of the noted channels, depending on its position in the bore.
2. Discussion of the Prior Art
A device of the above-mentioned kind is disclosed in U.S. Pat. No. 4,042,310. In the device described in this patent, the plunger directed towards the first end of the bore is put into connection with the low-pressure port by the supply of high pressure oil from the oil separator of the screw compressor on the high-pressure side thereof. Upon movement of the plunger in this direction, a helical spring is tensioned and endeavours to pull the plunger in a direction towards the other end of the bore (i.e., the high-pressure end of the screw compressor). The device as initially defined may be formed in a number of different ways. Thus, the plunger may be driven towards the low-pressure end of the bore by means of high pressure oil, thus compressing a helical spring which is acting to urge the plunger towards the other end of the bore. In both cases mentioned, oil is drained from the bore to the low-pressure side of the screw compressor. Unloading devices of the kind described operate satisfactorily in the case of screw compressors which are acting to compress gases such as NH.sub.3 or air, which dissolve only to a very small extent in the lubricating oil which circulates in the screw compressor. When the screw compressor is used for compressing refrigerants such as R.sub.22 (CH C1F.sub.2) or R.sub.12 (CC1.sub.2 F.sub.2), however, these refrigerants are dissolved to a relatively large extent in the lubricating oil so that when a lubricating oil with a high content of dissolved refrigerant is supplied to the bore to displace the plunger, the reduction in pressure which takes place causes the refrigerant to come out of solution in gaseous form. Thus, the presence of the refrigerant in the oil gives rise to an uncontrolled volumetric increase in the bore, which displaces the plunger further towards the low-pressure side than was intended. The regulation of the volume capacity of a screw compressor under these circumstances is therefore imprecise and thus generally unsatisfactory.